The present invention relates to an apparatus for automatically measuring the characteristics of an optical system, such as the spherical refractive power, the cylindrical refractive power and the orientation of the cylinder axis, and the prismatic refractive power and the orientation of the prism base. The principle and the examples of the present invention will hereinafter be described mainly with respect to a measurement for such characteristics of a spectacle lens. However, this does not necessarily mean that the present invention is applicable only to a so-called lensmeter which is adapted to measure the above characteristics of a spectacle lens, but the present invention is broadly applicable for measurements of the characteristics of a lens optical system used in optical instruments in general.
In recent years, various proposals have been made in the field of automatic lensmeters for automatically measuring the optical characteristics of a spectacle lens, such as the spherical refractive power, the cylindrical refractive power and the orientation of the cylinder axis and so on. For example, U.S. Pat. No. 3,880,525 discloses an apparatus in which a parallel light beam is projected through a lens along the optical axis of the apparatus and the optical characteristics are determined by the deflection of the light which has passed through the lens. For the purpose, the apparatus includes a mask located behind the lens and having small apertures which are offset from the optical axis of the lens, and a detecting plane which is spaced apart by a predetermined distance along the optical axis from the mask so that the locations on the detecting plane of the projections of the apertures are detected. The locations thus detected are compared with the locations of the apertures on the mask to calculate the direction and the amount of deflection of the light beam which has passed through the lens. In order to obtain adequate information, the mask must be provided with at least three such apertures.
The apparatus as proposed by the U.S. Patent is considered as being disadvantageous in that it is required to determine exactly which one of the projections on the detecting plane corresponds to each specific one of the apertures in the mask. Further, the apertures in the mask must be in a two-dimensional arrangement so that the light beams which have passed through the apertures in the mask are not coplanar with each other. Thus, a two-dimensional scanning is required at the detecting plane and the apparatus therefore becomes expensive as a whole. Complicated and expensive operation circuits are required because it is necessary to solve five simultaneous equations based on the informations derived from the locations of at least three apertures. The apparatus has a further significant disadvantage in that the image detection and measurement are disenabled by foreign matters such as dust on the lens to be tested or the detecting plane since the images are detected in the form of a spot-like image.
In order to solve the problems of processing bulky informations inherent to such two-dimensional detection, the U.S. Pat. No. 4,180,325 proposes to pass the light beam from the mask through a rotatable disc having a special pattern comprised of transparent and opaque portions. The disc pattern functions to intermittently interrupt the light beams so that the light beams arrive at the detecting plane respectively at different timings to thereby eliminate the necessity for discriminating the light beams. However, in this apparatus, the pattern on the disc is very complicated and the detection of the angular position of the disc is of a significant importance. Therefore, serious problems are encountered in providing an accurate pattern on the disc and detecting the angular position of the disc.